The present invention is generally related to the fabrication of electronic devices and, more particularly, to the fabrication of electronic devices that incorporate one or more layers of tungsten in their structure.
The trend in decreasing device sizes in the production of integrated electronic circuitry is continuing in association with a need for high performance and reliability. One aspect of development relates to the structure of the gate electrode of a transistor such as a MOSFET in these devices. In the past, it has been acceptable to use a polysilicon stack that includes doped polysilicon. Unfortunately, for upcoming technology nodes where silicon oxide gate dielectric is replaced by dielectric materials having a high k dielectric constant, the polysilicon gate stack is not likely to be acceptable due to the mismatch of work functions between the gate dielectric and the electrode materials. The consequence of this mismatch causes the polysilicon gate stack to exhibit a resistance that is unacceptably high. The current trend, therefore, is to incorporate the use of new materials with improved conductivity. One example of an acceptable material is tungsten. Unfortunately, however, many process regimes that are currently in use are not readily applicable to the formation of a gate electrode structure that includes tungsten or even tungsten in some combination with polysilicon. As one example, for a photoresist (PR) strip process on a tungsten gate structure using plasma reactors, it has been found that a conventional oxygen plasma causes serious tungsten oxidation which leads to a degradation of device performance. Accordingly, it is considered that one of ordinary skill in the art would be motivated to avoid the use of oxygen, in order to avoid this oxidation. As an alternative, one might use a reducing chemistry including hydrogen to prevent the oxidation. Hydrogen itself, however, strips PR at an impractically slow rate, and it may introduce safety issues. Moreover, it is believed that many prior art process regimes which utilize hydrogen gas as a plasma constituent are limited as a result of employing forming gas to provide the hydrogen gas. Forming gas typically includes 4% hydrogen gas and 96% nitrogen gas. While the use of forming gas reduces risks associated with the flammability of hydrogen gas and increases PR strip rate, it is believed that the essentially vast amount of nitrogen gas that carries the hydrogen gas introduces significant limitations with respect to processing results that have been demonstrated in the prior art, as will be discussed at an appropriate point hereinafter.
In view of the foregoing, it is considered that there remains an unfulfilled need with respect to fabrication of integrated devices that incorporate tungsten. The present invention is considered to resolve the foregoing concerns while providing still further advantages.